Program dtgexc_example
! DTGEXC Example Program Text
! Copyright 2017, Numerical Algorithms Group Ltd. http://www.nag.com
! .. Use Statements ..
Use lapack_example_aux, Only: nagf_file_print_matrix_real_gen
Use lapack_interfaces, Only: dtgexc
Use lapack_precision, Only: dp
! .. Implicit None Statement ..
Implicit None
! .. Parameters ..
Integer, Parameter :: nin = 5, nout = 6
Logical, Parameter :: wantq = .False., wantz = .False.
! .. Local Scalars ..
Integer :: i, ifail, ifst, ilst, info, lda, ldb, ldq, ldz, lwork, n
! .. Local Arrays ..
Real (Kind=dp), Allocatable :: a(:, :), b(:, :), q(:, :), work(:), &
z(:, :)
! .. Executable Statements ..
Write (nout, *) 'DTGEXC Example Program Results'
Write (nout, *)
Flush (nout)
! Skip heading in data file
Read (nin, *)
Read (nin, *) n
ldq = 1
ldz = 1
lda = n
ldb = n
lwork = 4*n + 16
Allocate (a(lda,n), b(ldb,n), q(ldq,1), work(lwork), z(ldz,1))
! Read A and B from data file
Read (nin, *)(a(i,1:n), i=1, n)
Read (nin, *)(b(i,1:n), i=1, n)
! Read the row indices
Read (nin, *) ifst, ilst
! Reorder A and B
Call dtgexc(wantq, wantz, n, a, lda, b, ldb, q, ldq, z, ldz, ifst, ilst, &
work, lwork, info)
If (info/=0) Then
Write (nout, 100) info, ilst
Write (nout, *)
Flush (nout)
End If
! The resulting reordered Schur matrices can differ by +- signs by
! multiplying rows and columns of Q and Z by -1. We will normalize here by
! making the diagonals and last column of B positive.
Call normalize(a, b)
! Print reordered generalized Schur form
! ifail: behaviour on error exit
! =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
ifail = 0
Call nagf_file_print_matrix_real_gen('General', ' ', n, n, a, lda, &
'Reordered Schur matrix A', ifail)
Write (nout, *)
Flush (nout)
ifail = 0
Call nagf_file_print_matrix_real_gen('General', ' ', n, n, b, ldb, &
'Reordered Schur matrix B', ifail)
100 Format (' Reordering could not be completed. INFO = ', I3, ' ILST = ', &
I5)
Contains
Subroutine normalize(a, b)
! .. Array Arguments ..
Real (Kind=dp), Intent (Inout) :: a(lda, n), b(ldb, n)
! .. Local Scalars ..
Integer :: i, j
! .. Intrinsic Procedures ..
Intrinsic :: max
! .. Executable Statements ..
! Last column of B positive
Do i = 1, n
j = max(1, i-1)
If (b(i,n)<0.0_dp) Then
a(i, j:n) = -a(i, j:n)
b(i, i:n) = -b(i, i:n)
End If
End Do
! Diagonals of B positive
Do i = 1, n - 1
If (b(i,i)<0.0_dp) Then
a(1:i+1, i) = -a(1:i+1, i)
b(1:i, i) = -b(1:i, i)
End If
End Do
End Subroutine
End Program